1. Field of the Invention
This invention relates to position determination of mobile users in a wireless communication system. More particularly, it relates to cellular communication systems in which position determination is provided as a low-complexity, augmentation service to an installed base of communication equipment.
2. Description of the Related Art
Mobile communication services have grown from infancy in the 1970s to become world wide in scope, having subscribers in numbers threatening to overcome the size of the fixed telephone base. First generation (1G) analog systems concentrated entirely on voice communications. Following an explosive growth of digital technologies and the rise of the Internet, there emerged a demand for mobile data services that has been met initially by second generation (2G) digital systems. More expansive, multi-channel, multi-media services are anticipated in third generation (3G) systems and beyond.
For a variety of commercial and safety reasons, a demand that the locations of mobile users be known to some accuracy, both by the mobile station and the network service provider, has arisen. Some of this demand has escalated to regulatory requirements, e.g. mobile enhanced 911 (E911), which mandates Automatic Location Identification (ALI) services beginning Oct. 1, 2001.
Position determination is sure to be a feature of mobile communications systems deployed in the future. Current provisioning of such services, however, requires that a position determination function be retrofitted to all US and many foreign operating networks to meet E911 requirements.
The original expectation, based on technology assessments, was that carriers would undertake network-based solutions for a mobile unit position, but none of the first and second generation cellular telephony systems explicitly incorporated means to do so. The FCC requirement for a network-based solution is 100 meters for 67% of calls and 300 meters for 95% of calls. With the development of commercial Global Positioning System (GPS) technology, it became clear that handset-based solutions would be possible as GPS capability became commonplace in handsets. The FCC requirements for a handset-based solution are twice as stringent as those for network-based solutions (50 and 150 meters for 67% and 95% of 911 calls, respectively).
As long as GPS operated in its intentionally degraded Selective Availability (SA) mode, it was incapable of providing the requisite accuracy. By presidential directive SA was terminated on May 2, 2000, permitting commercial GPS receivers to know their position within 10 s of meters.
Progress in meeting FCC E911, the most demanding ALI requirement, has been difficult to make. In the absence of any built-in ALI capability in cellular telephony signals, construction of a network-based solution required clever exploitation of existing signals or addition of new means. And even though a handset might know its position to sufficient accuracy from an internally or externally supplied post-SA GPS solution, no means to automatically couple such a solution into the return link to the base station was provided.
This is not to say that the problem has not been addressed. It has in fact already been approached from a variety of viewpoints. In one such conventional system, a mobile station in a cellular communications network estimates pseudorange to a plurality of base stations by measuring the arrival time of known signals, e.g. training sequences, included in the downlink communication signals transmitted by the base stations to mobile stations. With proper coordination between base stations and mobile stations some two-way ranging also may be performed. A differential time of arrival (DTOA) technique is used in such a conventional system to convert a set of pseudoranges to a position solution.
In another conventional system, each base station transmits, in addition to its communication signals, an embedded navigation signal that both provides mobile stations with a signal from which to extract signal arrival time and carries low-rate digital timing data derived from an external source, e.g. the Global Positioning System (GPS), to facilitate converting arrival time measurements to position estimates. In such a system the base station is equipped with a receiver (e.g., a GPS receiver) to provide the external timing reference. The base station also can be made to function as a pseudolite node in the external ranging system if it also provides navigation services. Proper execution of the position determination function in such systems, however, requires the external timing reference to be available at the base station, at the mobile station, or at both stations.
Theory and experiment indicate that conventional systems performing ranging using the communication signals above are not able to meet the most stringent of the position determination accuracy requirements, e.g. E911. Performance improvements are made possible by an embedded navigation signal, but the consequent dependence on external timing in the conventional systems creates an equipment burden at both the base station and the mobile stations that hampers backward compatibility. Further, such dependence is disadvantageous in that outage of the timing reference system impairs or disables the position determination function.
There is a long felt need to determine the position of mobile stations within a wireless communication network with a high degree of accuracy and without requiring substantial changes to the wireless communications network infrastructure.